In general, the present invention relates to the field of communication systems and more particularly, to multi-carrier wireless communication systems.
Orthogonal frequency division multiplexing (OFDM) is a well-known multi-carrier modulation scheme, which can be designed to operate in broadband channels with severe multipath. For broadband channels, a receive signal processing complexity of an OFDM system is typically lower than that of an equivalent equalized single-carrier system. However, OFDM requires channel coding in order to take advantage of the frequency diversity provided by multipath channels. Without channel coding, OFDM performs as if the channel is flat faded, leading to degraded (BER) performance. Therefore, most OFDM systems include channel coding. For best performance a code rate, where code refers to channel coding, must be low enough so that the channel code is capable of exploiting all of the channel diversity. As is known in the art, the diversity capability of the channel code decreases as the code rate is increased.
In a cellular wireless communication system, the downlink received signal quality, such as the signal-to-interference-plus-noise ratio (SINR), may vary significantly depending on the location of the user and other factors. For example, a user near a base station may experience a very high SINR, while a user at the edge of a cell will experience a very low SINR.
In order to accommodate the wide range of expected SINRs"" in the system, users with a high SINR can be assigned a high-order modulation and a high code rate, while users with a low SINR can be assigned a low-order modulation and a low code rate. However, this approach can still have two significant limitations. First, when the SINR is very high, the peak data rate provided by channel coded OFDM is limited by the code rate. Second, an edge-of-cell SINR can be extremely low with current aggressive cellular system frequency reuse plans; however, it is possible that even the lowest available modulation and coding rate (MCR), where coding refers to channel coding, requires a larger SINR than this edge-of cell value to provide reasonable BER performance. As a result, the system may have an unacceptably high outage probability.
One possible solution to these problems is to use multi-carrier code division multiple access (MC-CDMA) or spread OFDM (SOFDM). Code in MC-CDMA refers to spreading code. MC-CDMA typically performs better than OFDM for uncoded transmissions. But MC-CDMA suffers from self-interference if multiple spreading codes (multi-code) are transmitted over a frequency-selective (delay-spread) channel. The self-interference is due to the fact that the frequency selective channel destroys the orthogonality between the transmitted spreading codes.
Thus there is a significant need for a method and device for improving a communication system that overcomes the above disadvantages and shortcomings, as well as other disadvantages.